1. Field of the Invention
The present invention relates to a lighted switch illuminator used for operating electronic devices such as portable telephones, and its fabrication method.
2. Description of Related Art
A portable telephone employs lighted switches as a dial and functional switches. They are composed of key buttons illuminated by a backlight for facilitating manipulation on if a rather dark road at night or the like.
These electric devices including portable telephones have been made thinner and smaller with their increasing functions. In particular, thin-type portable telephones have been appearing, in which the thickness of their batteries occupy 70-80% of their total thickness. It is considered to be difficult to further reduce their thickness with maintaining the capacity of the batteries.
FIG. 23 is a perspective view of a conventional portable telephone. In this figure, the reference numeral 1 designates a front case, and 2 designates a rear case. These cases 1 and 2 constitute a main body case incorporating electronic devices such as a printed board on which ICs or LSIs are mounted. The reference numeral 3 designates a liquid crystal display (LCD) mounted on the front case 1, and 4 designates key buttons consisting of dial keys, function keys and the like disposed on the front case 1. The key buttons 4 are illuminated by backlight emitted from an illuminator not shown in this figure to facilitate their manipulation in a dark place. The reference numeral 5 designate a flip with its end rotatably fixed to the front case 1. Although its open state is shown in FIG. 23, it covers the key buttons when closed to protect them and prevent their misoperation. The reference numeral 6 designates a microphone mounted on the flip 5, 7 designates a speaker, and 8 designates a transmitting and receiving antenna.
FIGS. 24 and 25 are cross-sectional view of a portion of a lighted switch including one of the key buttons 4. The key button 4 is composed of a transparent material with numerals and marks printed on its face and with a protrusion 4a on its back. It is supported by an elastic material between the front case 1 and a printed board described below in such a manner that it can move in a vertical direction. The reference numeral 9 designates a film click board composed of a translucent plastics sheet, and 10 designates a hollow convex portion disposed such that it faces the back of the key button 4. The hollow convex portion 10 has a dome-like shape drawn out by pressing the film click board 9. The reference numeral 11 designates an electroluminescent (called EL for short below) light functioning as a light emitting source, which is composed of a base film 12, a transparent electrode layer 13, a light emitting layer 14, a dielectric layer 15, a rear electrode layer 16 and an insulation layer 17, which are successively stacked-on the back of the film click board 9. The reference numeral 18 designates a double-sided adhesive tape for bonding the back of the film click board 9 and the face of the EL light 11; 19 designates a switch electrode sheet, on which a switch circuit corresponding to a plurality of key buttons are printed; 20 designates upper contacts consisting of carbon ink printed on the back of the switch electrode sheet 19; 21 designates a printed board including electronic components constituting a transmitter and receiver, radio circuits or the like mounted or printed thereon; 22 designates a spacer interposed between the printed board 21 and the switch electrode sheet 19; and 23 designates a lower contact provided on the printed board 21 such that it faces the upper contact 20.
Next, the operation of the conventional lighted switch illuminator will be described.
With depressing the key button 4 in the state as shown in FIG. 24, the protrusion 4a of the key button 4 presses the hollow convex portion 10 of the film click board 9 and changes its form. When the hollow convex portion 10 changes its form beyond a certain level, it inverts its shape to protrude to the opposite side, thereby depressing the EL light 11 and the switch electrode sheet 19, and making the upper contact 20 and lower contact 23 contact to establish conduction. This state is shown in FIG. 25. When the upper contact 20 and lower contact 23 are connected, a switch circuit printed on the switch electrode sheet 19 generates a control signal so that a CPU governing the functions of the portable telephone, for example, detects that one of the key buttons 4 has been depressed, and carries out corresponding display on the liquid crystal display 3.
The switch has a lighted switch structure, in which the light emitted from the light source EL light 11 illuminates the back of the key button 4.
More specifically, the EL light 11, composed of the base film 12, transparent electrode layer 13, light emitting layer 14, dielectric layer 15, rear electrode layer 16 and insulation layer 17 which are stacked in this order, emits light when electric field excitation and collision of electrons to the light emitting layer 14 are induced by applying an ac voltage between the transparent electrode layer 13 and the rear electrode layer 16.
The operation feeling of the key button will be explained with reference to FIGS. 26-28.
FIGS. 26-28 are characteristic diagrams whose vertical axis represents the load and horizontal axis represents the stroke when manipulating the key button 4. In these diagrams, P1 and P1' each designate a load immediately before the film click board 9 is inverted while it is depressed by the key button 4 and changing its form, S1 designates a stroke of the key button 4 at that time, P2 and P2' each designate a load at the instant when the film click board 9 is inverted and the upper contact 20 contacts the lower contact 23, and S2 designates the stroke at that instant.
One of the factors determining the operation feeling of the key button is a clicking feeling, which is considered to be improved with an increase of a click ratio (P1-P2)/P1, and a decrease of the difference S2-S1.
The conventional lighted switch as shown in FIGS. 24 and 25 cannot cause the upper contact 20 to contact the lower contact 23 unless the EL light 11 and the switch electrode sheet 19 are warped as well as the film click board 9. In other words, it has the characteristic as shown in FIG. 28 obtained by superimposing the characteristic curve of the film click board 9 as shown in FIG. 26 over the characteristic curve of the EL light 11 and the switch electrode sheet 19 as shown in FIG. 27.
Accordingly, the original click ratio (P1-P2)/P1 of the film click board 9 is reduced to (P1'-P2')/P1' owing to the EL light 11 and switch electrode sheet 19, and hence a good clicking feeling cannot be achieved in the conventional lighted switch as shown in FIGS. 24 and 25.
As described above, the conventional lighted switch is assembled by bonding the separate film click board 9 and the EL light 11 of the illuminator functioning as the light source by using the double-sided adhesive tape 18. As a result, its thickness h remains large as indicated in FIG. 24, which poses a problem in that the external dimensions of the lighted switch and the electronic equipment using it cannot be made thinner and smaller.